(1) Field of the Invention
(2) Description of the Prior Art
The present invention relates to a combustion unit for combusting a liquid fuel and to a system for generating power comprising such a combustion unit.
In the combustion of liquid fuel, in particular engine fuels such as petrol, kerosine, diesel and methanol, it is important that at the time of the combustion the fuel is present in the smallest possible particles. The smaller the fuel particles, the more homogeneous a combustion results. A more homogeneous combustion is associated with less soot formation and soot emission as well as less CO formation and emission.
It is therefore the object to introduce the smallest possible fuel droplets into the combustion chamber. Known combustion units are characterized by assorted additional means for obtaining the smallest possible fuel droplets in the combustion chamber at the moment of combustion.
The present invention has for its object to provide a combustion unit for combusting liquid fuel which is provided with means for carrying into the combustion chamber very small liquid fuel particles (median size  less than 5 xcexcm, generally  less than 3 xcexcm, preferably  less than 2 xcexcm, such as 1.2 xcexcm). Thus, a sufficient supply of these very small liquid fuel particles can be ensured and the means for obtaining these very small liquid fuel particles have a relatively simple construction and can be added in relatively simple manner to existing combustion units.
This is achieved according to the invention with a combustion unit for combusting a liquid fuel, comprising a fuel inlet, an air inlet and a flue gas outlet which are connected to a combustion chamber for combusting the fuel, wherein the fuel inlet is connected to at least one explosion atomizing unit which is disposed and adapted such that atomized fuel fragments due to gas formation in the atomized fuel.
The means for realizing these very small liquid fuel particles consist of explosion atomizing units.
All known types of atomizer can in principle be used in the explosion atomizing unit. Swirl atomizers, slot atomizers, hole atomizers, rotating plate or bowl atomizers and optionally pen atomizers are for instance suitable. All that is important is that the atomizer generates droplets or a film of liquid fuel to the gaseous medium under changed conditions such that explosion atomizing then occurs. Explosion atomizing entails the liquid fuel entering the combustion chamber under conditions such that as a result of the pressure drop over the atomizer boiling or gas bubbles occur in the droplets or film of the liquid fuel. That is, gas formation occurs in the liquid fuel. This so-called flashing or precipitation results in the droplets or film of fuel exploding or fragmenting due to the sudden partial boiling or gas precipitation. This fragmentation results in very small droplets of fuel being generated in the gaseous medium. The median dimension of fuel particles amounts after fragmentation to less than 5 xcexcm, generally less than 3 xcexcm, preferably less than 2 xcexcm, for instance 1.2 xcexcm.
It is noted that the explosion atomizing unit does not have to deliver the atomized liquid fuel directly into the combustion chamber. It is sufficient that the generated fuel droplets finally enter the combustion chamber without an undesirably large droplet growth having taken place as a consequence of coalescence.
The invention allows the use in the atomizing means of all types of atomizers insofar as these can result in particles with said median size after fragmentation. It is important in this respect that the explosion atomizing units are disposed and adapted such that the atomized fuel fragments through gas formation in the atomized fuel.
Use is preferably made of an explosion swirl atomizing unit which is provided with swirl atomizers. In such a known swirl atomizer a swirling movement is imparted to the liquid fuel in a swirl chamber. The swirling fuel exits from an outlet opening. It has been found that the thickness of the exiting layer of fuel is a fraction (for instance 10%) of the diameter of the outlet passage. Due to the subsequent explosion fragmentation, particles are obtained (depending on the pressure drop, temperature and passage diameter) with a median dimension of 5 xcexcm or smaller.
It will be apparent that in order to realize this fragmentation it is important that the conditions (and particularly change in conditions) under which the liquid fuel is atomized are optimal for fragmentation. Important conditions for flash-fragmentation are the temperature of the fuel, the atomizing pressure under which the fuel is atomized, the pressure drop during exit and the passage diameter. It is therefore recommended that the explosion atomizing unit comprises means for adjusting the temperature of the evaporating agent and/or the atomizing pressure.
In the case of retrofit of the above stated combustion unit, it is possible to integrate a configuration of a number of explosion atomizing units into a new or modified air inlet, or to have these explosion atomizing units debouch directly into the combustion chamber. By orienting the outlet passage of each explosion atomizing unit it is possible to atomize the fuel such that it is optimal for the forming of the mixture of fuel and air for combustion. Particularly recommended are swirl atomizers and slot or hole atomizers since these have a very simple construction, can be readily miniaturized and built into existing combustion units. Very large numbers of explosion atomizing units can thus be incorporated without too many modifications of an existing combustion unit, which offers great freedom in the choice of fuel flow rate to the combustion chamber. Retrofit of existing combustion units thus results in combustion units which can be converted at lower cost and which nevertheless realize a greatly improved combustion with a lower soot and NOx emission.
As stated, liquid fuel can be applied as fuel. The liquid state herein refers to the state of the fuel at the temperature and pressure prevailing in the fuel inlet. This means that fuels can be used which are gaseous in ambient conditions. Fuels such as diesel and petrol have a boiling range. This means that in order to realize the explosion atomizing a temperature must be chosen from the boiling range such that a significant flash effect occurs. For diesel oil a temperature can be chosen of 350xc2x0 C. For kerosine/petrol a lower fuel temperature can be chosen (250/150xc2x0 C.). A higher fuel temperature, such as 400xc2x0 C., can be chosen for low-speed marine diesel engines. It is noted however that these temperatures can vary depending on the pressure applied and optional fuel additives which have a positive effect on the explosion atomizing. It will be apparent that in order to realize an optimal explosion atomizing a combustion unit will preferably be equipped with means for adjusting the temperature and the atomizing pressure of the fuel.
If in further preference the temperature-adjusting means adjust the temperature of the evaporating agent around or to the critical temperature, the evaporating agent acquires a surface tension of practically or equal to 0 N/m2. This means that no further or little atomizing energy is required to atomize the liquid, whereby the droplet size will become extremely small (a median droplet dimension to 0.1 xcexcm is possible here) and the use of other agents to decrease the surface tension can optionally be dispensed with.
In addition to said physical conditions for fragmentation, it is also possible to enhance fragmentation by chemical or physical additives to the fuel. It is therefore recommended to add agents to the fuel which reduce the surface tension of the fuel and thereby decrease the energy required for fragmentation. Detergents and the like can be used as surface tension-reducing agents. Preferred are those surface tension-reducing agents which do not remain only on the surface of the fuel droplet but which are distributed almost homogeneously through the fuel (droplet or film). It is thereby not required that, after atomizing and prior to fragmentation, the surface tension be reduced to a lesser extent as a result of diffusion. In these conditions it is recommended to use fatty acids, particularly shorter fatty acids and optionally alcohols such as methanol and ethanol. These latter agents are particularly recommended because of a relatively low boiling point and good combustion. Thus is avoided that the combustion process is affected in a negative sense by these additives.
According to another embodiment the fuel contains combustible and/or vaporizable substances which either reduce the surface tension of the fuel or enhance the gas formation in the fuel as a result of the pressure drop over the atomizer. Combustible and/or vaporizable substances can particularly be used here which have a boiling point lower than the boiling point of the fuel. This should be understood to mean that in the case of a boiling range of the fuel, and optionally of the evaporating agent, these ranges are chosen such that the evaporating agent makes an essential contribution to the gas formation and ultimately the fragmentation of the fuel. When a number or mixture of evaporating agents are used, the vaporizable substances with the lowest boiling point will suddenly evaporate first and form boiling bubbles due to the pressure drop when passing through the explosion atomizing unit, whereby liquid fuel explodes or fragments into small droplets. A mixture can for instance be used of diesel oil as fuel and water as evaporating agent. Superheated evaporating agent (water) can also be used as evaporating agent (for instance water) and can be applied particularly in oil-fired boilers for generating steam. In which case fuel and superheated water can also be introduced separately into the boiler by explosion atomizing. The additional advantage is realized here that through the evaporation of the water the temperature of the mixture is lower prior to combustion, during combustion and after combustion, which enhances the performance of the combustion unit and reduces the emission of CO and NOx.
The combustion unit can be applied in a combustion engine, for instance a gas engine, petrol engine or diesel engine. In addition, the combustion unit can be incorporated in a system for generating power which comprises a compression device driven by a gas turbine and the combustion unit according to the invention in which fuel and air compressed by the compression device are combusted and fed to the gas turbine.
It will be apparent that it is very advantageous in this respect if explosion atomizing units are used in the compression device to atomize determined evaporating agents with a comparably higher evaporation energy (for instance water). A quasi-isothermal compression is hereby obtained whereby the compression work is reduced considerably. In the case the combustion unit is provided with a compression chamber and a combustion chamber, the explosion atomizing unit for the fuel can be connected to the combustion chamber and an explosion atomizing unit for evaporating agent for the purpose of evaporation cooling can be connected to the compression chamber.
During the compression stroke and the firing stroke of the combustion engine an optional quasi-isothermal compression, and in any case an optimal combustion, can thus take place. It is further recommended in the case of evaporation cooling that between a compression chamber and a combustion chamber of the combustion engine at least one pressure vessel is received which is in heat-exchanging contact with a combustion gas outlet of the combustion engine. It is thus possible in the cool compressed air to recuperate heat from the heat of the flue gases. If the residence time in the pressure vessel is too short, a number of pressure vessels can be applied in parallel or a relatively large pressure vessel in combination with a number of combustion chambers.
Mentioned and other features of the combustion unit and the power-generating system according to the invention will be further elucidated hereinbelow with reference to a number of embodiments which are given by way of example without the invention having to be deemed limited thereto.